1. Field of the Invention
The present invention relates generally to improvements in the construction of pump assemblies, and more particularly but not by way of limitation, to improvements in shaft to shaft seal alignment in such pumps assemblies.
2. Discussion
Potted pump assemblies are used extensively in this country and abroad in various fluid transfer applications. Potted pump assemblies are generally classified as a type having a motor contained within a hermetically sealed housing for separation from the environment in which the pump assembly must operate. Potted pump assemblies are commonly powered by an electrical motor which drives a pump. It is necessary to provide sealing integrity in the location where the rotatable shaft of the motor extends from the housing to engage and operate the externally mounted pump.
A potted pump assembly is used whenever it is necessary to provide a separating seal between the motor and the environment in which the pump assembly operates. One example is a pump assembly operating in an explosive environment, where it is necessary to prevent the exposure of electrical arcing to the environment. Further applications for a potted pump assembly are in a clean room environments, and in food processing environments, where it is necessary to prevent exposure of contamination such as oil within the motor housing from entering the environment.
One type of potted pump assembly is a submersible pump assembly, which operates while submersed in the fluid being pumped. Submersible pump assemblies must meet various industrial and household needs. A common industrial use is in coolant or lubrication fluid transfer in manufacturing or processing equipment. Common household uses find submersible pump assemblies used in removing unwanted water, as in basement flooding; submersible pump assemblies are also widely used in decorative fountains and water gardens.
This market for potted pump assemblies demands the manufacture of a relatively lightweight and inexpensive pump assembly that is reasonably efficient to operate. Furthermore, in a consumer market, pleasant styling along with an attractive and durable color finish are often what appeal to the customer. These product feature requirements have lead to the manufacture of these pump assemblies from primarily plastic components. Plastic mold injected parts provide a less expensive design, and plastic part construction makes it possible for many secondary operations, such as drilling and tapping of holes, to be completely eliminated. This reduces both assembly labor costs, and operational burden costs. Plastic parts made with pigmented raw material provide superior color and gloss retention reliability as compared to painted parts. Painting, too, requires secondary operations resulting in a more expensive design. Another advantage to plastic construction is the corrosion resistance superiority of plastics as compared to metals.
Parts made by the plastic mold injection process present certain challenges, however, in the design and assembly of pump components. Warpage and draft are common characteristics of plastic mold injected parts, contributing to component part to part variation. Design tolerances in some functionally critical mating parts, as in the motor shaft to the shaft seal, require precise parts with minimal variation to prevent secondary fitting operations. Correct alignment of the shaft to the shaft seal is dependent on precise positioning of the motor relative to the shaft seal bore.
Misalignment of the motor to the shaft seal bore presents difficult assembly problems. It is not unusual for the standard assembly procedure to consist of trained assemblers performing hand fit functions such as trimming, shaving, press fitting, and striking the components to hand fit and force the components. This results in the need for a relatively high degree of craftsmanship by the assemblers, resulting in difficult challenges in managing the acceptable job skills necessary to properly assemble a pump. Individually modified and hand fitted parts also results in the production of pumps lacking consistent operating performance. The described assembly procedure is an expensive way to operate, and is likely to produce unreliable pumps with unpredictable operating performance.
Misalignment of the motor shaft to the shaft seal can result in premature wear of either the motor shaft or the shaft seal. Excessive wear can lead to fluid leakage past the shaft seal, resulting in motor failure or an environmental risk. Misalignment of the motor shaft to the shaft seal can also result in excessive frictional forces, which can produce heat buildup at a rate faster than that which can be dissipated. This excessive heat can also shorten the life of the motor.
Fluid leakage past the shaft seal into the motor housing can also present a safety hazard. Liquids in contact with electrically live connections can conduct voltage which could produce a catastrophic safety danger to a person in contact with the liquid. Remedies to minimize the risk of this occurrence are well known in the art, and include the use of ground fault circuitry, and double insulation techniques. Nevertheless, there is a need in the industry for a pump with a more reliable shaft seal assembly construction, to prevent the described reliability problems and associated safety risks.